近年來由於設備價格不斷下降，加上其安裝的簡易性及配置上之高度彈性，使得無線區域網路的建置與應用能快速的發展，然而，由於訊號的衰減、干擾、網路壅塞等問題所造成的傳輸錯誤與不穩定的傳輸頻寬，使得在無線區域網路上進行視訊資料的傳輸往往都需要有適當的錯誤保護機制設計，除此之外，在多個基地台(base stations)間進行漫遊時所造成的資料遺失，對視訊串流應用也是另外的一個挑戰。在無線網路串流傳輸視訊資料所遭遇的封包遺失問題將導致畫面品質嚴重惡化，由於視訊編碼標準採用運動補償預測編碼技術，封包遺失不僅降低目前畫面品質，而且將導致錯誤擴散到隨後的畫面，使用錯誤保護編碼(error resilient coding)/轉換編碼(transcoding)工具增加冗餘(redundancy)的資訊去保護視訊資料是處理封包遺失問題的一般做法。
在本論文中我們針對無線網路上的視訊串流應用提出有效的錯誤保護技術，同時也考慮到在無線網路上進行漫遊及群播問題的解決，一種能應用在三層次(three-tier)串流架構中有效的兩階段(two-pass)錯誤保護轉換編碼方法，能增強遞送到可移動性終端設備視訊串流資料之錯誤保護的強韌性，首先，為了視訊資料的傳輸，我們結合了可調式編碼(scalable coding)及多重敘述編碼(multiple description coding)技術，提出一種多重敘述細緻可調式的編碼器(MD-FGS: multiple-description fine granularity scalability)去增強資料錯誤的保護能力，同時又能維持時間上的估測編碼效能，對於可移動性的終端使用者在無線網路上的漫遊需要，我們也提出了一個有效的軟式交握(soft handoff)方法，以減輕漫遊傳輸通道轉換時所造成封包遺失對視訊品質破壞的影響；接下來，我們也提出了一種內容導向(content-aware)的兩階段轉換編碼技術，在這編碼技術的第一階段中，編碼轉換器(transcoder)從資料流(bitstream)中先萃取了視訊內容的特徵，同時根據反饋(feedback)的通道統計資料估測當下網路的品質，在第二階段，根據這些視訊內容的特徵及估測的通道狀況，編碼轉換器將適切的分配來源編碼(source coding)及通道編碼(channel coding)資源的安排以進行錯誤保護能力的轉換編碼。
基於這個兩階段的錯誤保護轉換編碼方式，我們首先提出了一種具有利益追蹤(PT: profit tracing)的有優先權內容導向內部更新(CAIR: content-aware intra-refresh)技術(CAIR-PT)去保護最重要的資料區塊(macroblock)以對抗封包遺失的問題，一種最大最小化(minimizing the maximum)的準則也被採用以改善群播(multicasting)應用中使用者被照顧的公平性(fairness)，接下來，我們也提出了一種以內容為導向的可適性重傳次數限制(CA-RLA: content-aware retry limit adaptation)方法，在這個方法中，我們根據每個封包預先估算的遺失影響特性，對於遺失的封包適切的決定其重新被傳送的安排，如此的做法在解碼端可允許的傳輸延遲限制(delay constraint)下，使得影像群組(group of picture)的總錯誤擴散(error-propagation)影響能得到最小的最佳化，我們所提出的錯誤保護轉換編碼架構除了可以減緩錯誤擴散的影響，同時能夠維持低的計算複雜性，使得該架構非常適合於即時的視訊串流應用。
實驗的結果顯示，當封包遺失時我們所提出的MD-FGS編碼方法除了可以有效減輕錯誤漂流(drift)的問題，也能夠增加在時間軸上的估測編碼效能，同時在漫遊時的短暫過程中使用了多重分散路徑(path diversity)傳輸方法可以有效的提供錯誤保護的能力；而我們所提出的CAIR-PT編碼方法，在視訊群播的應用上可以降低終端使用者間影像品質損失的平均(mean)數及其變異(variance)量，以維持其公平性；此外，我們所提出的CA-RLA可以保證封包能準時的到達接收端以進行解碼播放，如此可以比傳統固定重傳次數限制的方法更能改善接收到的視訊品質。

With low cost, easy deployment, and flexible connectivity, the deployments of wireless networks have become widespread and fast-growing everywhere. However, the challenges of coping with the time-varying error rate and fluctuating bandwidth of a wireless network bring out the need of error resilient video transport. The packet loss problem may lead to serious video quality degradation, which not only affects the quality of current frame, but also leads to error propagation to subsequent frames due to the motion-compensated prediction and entropy coding used in current standard video coders. Using error resilience coding/transcoding tools to add redundancy to protect video data is a common strategy to handle the problem of packet loss. Besides, video transport over wireless networks usually requires retransmissions to successfully deliver video data to a receiver in case of packet loss, leading to increased delay time for the data to arrive at the receiver. Delay constraint is, however, one of the most important requirements in real-time applications since a video packet arriving later than the presentation time will become useless for the client.
In this thesis, we propose efficient error protection techniques to address the problems of video transport in a three-tier streaming framework which involve hybrid wireline and wireless networks. We first propose a multiple-description fine granularity scalability (MD-FGS) coder that combines scalable coding with multiple description coding (MDC) to enhance error resiliency while retaining good temporal prediction efficiency for video transport. An adaptive soft handoff algorithm is also proposed to effectively mitigate video quality degradation caused by transient channel switching when a mobile client roams in wireless networks.
We then propose a two-pass error-resilience transcoding scheme based on adaptive intra-refresh for inserting error resilience features to a compressed video at the intermediate transcoder of a three-tier streaming system. The proposed transcoder adaptively adjusts the intra-refresh rate according to the video content and the channel’s packet loss rate to protect the most important macroblocks against packet loss. In this work, we consider the problem of multicast of video to multiple clients with disparate channel loss profiles. We propose a minimax loss rate estimation scheme to determine a single intra-refresh rate for all the clients in a multicast group. For the scenario that a quality variation constraint is imposed on the users, we also propose a grouping method to partition a multicast group of heterogeneous users into a minimal number of sub-groups to minimize the channel bandwidth consumption while meeting the quality variation constraint.
Finally, we propose a cross-layer content-aware retry limit adaptation scheme for video streaming over IEEE 802.11 wireless LANs (WLANs). In our method, video packets of different importance are unequally protected with different retry limits at the MAC layer. The error propagation effect of each packet is estimated to guide the determination of its retry limit. More retry numbers are allocated to packets of higher loss impact to achieve unequal error protection. Our scheme also analyzes the backoff time for each retry and then takes into account the estimated backoff time for retransmission scheduling.
Experimental results demonstrate that the proposed MD-FGS coder can increase the temporal prediction efficiency without introducing severe drifting error when packet loss occurs. Furthermore, the use of path diversity as temporary step for roaming can provide significant benefits for error protection. The proposed error resilience transcoder effectively reduces the mean and variance of penalty distortion as well as achieves fairness among clients in video multicasting with heterogeneous channel conditions. Furthermore, the proposed retry limit adaptation scheme can effectively mitigate the error propagation due to packet loss and assure the on-time arrival of packets for presentation.

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